The present invention is drawn to a novel SCALPEL mask and method utilizing the SCALPEL mask and Scattering Angular Limited Projection Electron Lithography for forming semiconductor devices.
One goal in modern semiconductor fabrication is to improve the density of active elements provided on a single semiconductor die, thus increasing the number of die per wafer. As is known in the art, very large scale integration (VLSI) has evolved into ultra-high large scale integration (ULSI). In order to improve density without overly increasing die size, there is ongoing investigation in decreasing further the critical dimension (CD) of active elements provided on the semiconductor die. Lithographic techniques are typically used in the formation of multi-level circuits on a semiconductor die. Currently, lithographic techniques take advantage of I-line (365 nanometer) and deep ultra-violet (DUV, 248 nanometer) energy sources. By decreasing wavelength of the energy utilized in these lithographic techniques, smaller CD's may be realized.
Accordingly, smaller wavelength, higher energy sources have been investigated, including DUV at 193 nanometers, X-ray, Ion projection, EUV (extreme ultra-violet), as well as scattering angular limited projection electron lithography (SCALPEL). Of these techniques, SCALPEL has been identified as a particularly promising technology for achieving ever finer CD's with high throughput.
Turning to FIG. 1, the basic principles of SCALPEL are illustrated. As shown, a mask 10 having a patterned scattering layer is provided on membrane 12, through which an electron beam is projected, as represented by the arrows. The patterned scattering layer 14 has a higher scattering power than that of the membrane 12, provided by a difference in atomic numbers between the patterned scattering layer 14 and the membrane 12. Particularly, the patterned scattering layer has a higher atomic number than that of the membrane. The scattering effect of the electron beam through portions of the mask is illustrated in FIG. 1. As shown, those portions of the electron beam that pass through the patterned scattering layer 14 tend to be scattered to a greater extent as compared with those portions that pass between patterned portions (i.e., unpatterned portions) of the scattering layer 14.
As shown, the electron beam passes through the mask, is focused through an electron focusing system, represented by lens 20. The electron beam then passes through back focal plane filter 30 having an aperture that is provided to permit passage of those portions of the electron beam that were not scattered by the patterned scattering layer of the mask 10. The electron beam is then projected onto a semiconductor wafer 40 having a plurality of die 42 and a resist layer 44 formed thereon by conventional techniques such as by spinning-on. The electron beam forms a high contrast image including areas of high intensity formed by those portions of the electron beam that pass through unpatterned portions of the mask 10, and areas of relatively low intensity formed by those portions of the electron beam that pass through the patterned areas of the mask 10. In this way, a high-resolution image may be projected onto the resist layer, which is then developed to form a patterned resist layer. Thereafter, the material exposed through the patterned resist layer may be etched using an appropriate etchant. It is noted that the power of the system may be adjusted so as to provide a 3-5.times. reduction in image size, typically 4.times..
Turning to FIGS. 2-1 to 2-4, a typical process for forming a mask for SCALPEL use is illustrated. First, a silicon substrate 102, such as on the order of 400-800 microns and 100-300 millimeters in diameter, is provided. The substrate 102 is formed of monocrystalline silicon, but other monocrystalline materials may be utilized. The substrate 102 is subjected to an LPCVD process to form silicon nitride bottom layer 100 and membrane layer 104 on opposing major surfaces of the substrate. Layers 100 and 104 are typically on the order of 1,000 angstroms of thickness. Thereafter, an etch stop layer 106 is deposited upon membrane layer 104, typically on the order of 100 angstroms in thickness. A scattering layer 108 is then provided on the etch stop layer 106. Typically, the etch stop layer is formed of Cr, while the scattering layer may be formed of any one of several high atomic number species, such as W.
Turning to FIG. 2-2, an opening 103 is etched in the substrate 102, thereby leaving window portion 109 of relatively small thickness that spans opening 103. A resist 110 is coated and patterned on scattering layer 108, as shown in FIG. 2-3, and scattering layer 108 is etched so as to form patterned scattering layer 108', as shown in FIG. 2-4. According to the final structure shown in FIG. 2-4, the electron beam may pass through the entirety of the window portion 109, and is largely blocked by the substrate 102 along un-etched portions thereof.
While SCALPEL technology has been demonstrated to provide improved resolution over conventional techniques, including I-line and DUV processing, the present inventors have recognized numerous deficiencies with conventional SCALPEL technology, particularly the mask utilized therefor.
It is recognized in the art of lithography that masks typically have to be cleaned after repeated use. However, the mask utilized in SCALPEL technology represents numerous difficulties. Particularly, the present inventors have discovered that it is relatively difficult to clean the mask, due to the topology of the mask. For example, relatively small particles may drop onto the mask and be trapped between adjacent patterned lines, that is, between patterned and unpatterned regions of the mask. In this regard, it has been found that state of the art cleaning techniques have not been sufficient to remove such particles without damaging the scattering layer on the mask. For example, the conventional aggressive RCA clean has not proven to be an effective cleaning procedure while maintaining the integrity of the mask. That is, conventional aggressive cleaning techniques utilize chemical species that undesirably interact with the materials of the mask, particularly the scattering layer thereof. In addition, because of the delicate nature of the membrane covering the openings in the substrate, physical agitation, such as ultrasonic agitation, is generally undesirable. Other cleaning techniques such as dry laser cleaning, and frozen ice cleaning are likely to be ineffective to clean adequately the mask, particularly to remove contaminants provided between patterned and unpatterned portions of the mask.
The present inventors have also recognized problems with pin hole defects in the mask, particularly in the membrane portions of the mask. Such pin hole defects result in reduced yield due to a tearing or breakage of the membrane portions at the site of the pin hole defect, due to the membrane portions being in tension.
Further, as noted above, the etch stop layer 106 is an essential component of the prior art to form an effective etch stop barrier during etching of the scattering layer. Absent such an etch stop layer, it has been found that undesirable roughening and pitting of the membrane layer 104 takes places during etching of the scattering layer, which undesirably weakens the membrane. However, it has been recognized in the art that it is desirable to decrease the overall thickness of the membrane to the extent possible. Accordingly, it is desirable to eliminate such an etch stop layer if feasible.
In consideration of the above disadvantages of the known SCALPEL mask, the present inventors have considered use of a pellicle-like structure, which are typically used in optical masks. As is known in the art, pellicles in optical systems are comprised of an optically transparent (e.g., glass) material spaced apart and above the mask. Use of the pellicle in an optical system is effective. It prevents contamination of the mask, and any fall-on defects or contamination on the pellicle are not imaged onto the resist. However, use of a pellicle in SCALPEL technology is not effective, since the pellicle increases the thickness of material through which the electron beam must pass, thereby reducing throughput, and increasing chromatic aberration. In addition, in SCALPEL technology, any contamination or fall-on defects deposited on the pellicle are nevertheless imaged onto the resist, unlike in the optical systems.
Accordingly, a need exists in the art for an improved SCALPEL technique that utilizes a mask that has improved properties, and which has membrane portions of substantially the same thickness or even reduced thickness with respect to the state of the art masks.